ox THE VARIOLITIC DIABASE OF THE FICETELGEBIRGE. 45 



5. The Yariolitic D-iabase of the Pichtelgebirge. By J. Walter 

 Gregory, F.G.S., F.Z.S., of the British Museum (Nat. Hist.). 

 (Read December 17, 1890.) 



Contexts. 



I. Introduction. 

 II. General Features of the Surface. 



III. The Variolitic Diabase. 



IV. The " Pseudocrystallites." 



V. The Relations of the Diabase. 

 VI. The Origin of the Variolitic Structure. 

 VII. Summary of Conclusions. 



I. IXTRODTJCTION. 



Amoxg the many interesting rocks described in 1874 by Prof, von 

 Giimbel in his ' Die paliiolithischen EruiDtivgesteine des Fichtel- 

 gebirges ' * was a variety o£ diabase crowded with small round, 

 apparently felspathic bodies. As these stood out on weathered sur- 

 faces like peas or pearls, he suggested for it the name of " Perl- 

 diabas." Seeing that this peculiar modification occurred around 

 the margins of the diabase masses, close to the contact with the 

 sedimentary rocks into which they were intruded, Prof, von Giimbel 

 regarded these pearl-like spheroids or " KnoUchen " as fragments of 

 the neighbouring beds caught up by the igneous rock, rounded by 

 the rolling and the fusion of the sharp angles, and baked till they 

 acquired the porcellanitic appearance that they now present. In 

 the following year, Prof, von Zirkel, in his well-known paper " Die 

 Structur der Yariolite " f, described the microscopic structure of 

 this rock, and, recognizing that the rounded spheroids are merely 

 ordinary spherules, he identified it as variolite, and compared it 

 with the typical varieties ; he objected alike to von Giimbel's name 

 and theory. 



Prof, von Zirkel's conclusions were confirmed by Prof. Posenbusch 

 in his detailed microscopic examination of the rock J, and he, in 

 accordance with the view taken by the French geologists of the 

 origin of the Savoy variolites, regarded these also as ordinary 

 " endomorphen Contactformen der Diabase " on the selvage of 

 intrnsive dykes. 



Prof, von Giimbers faith in his own theory stood firm against 

 these criticisms and this new evidence ; and in 1879, in the third 

 volume of his great work on Bavaria §, he re-enunciated his hypo- 



* P. 31 ; and X. Jahrb. 1376. pp. 42, 43. 



t Berichte k. siiehs. Gresell. Wissensch. vol. xxvii. (1875), pp. 209-220. 



I ' Mikroskopische Phvsiographie der massigen G-esteine' (1877), pp. 35S- 

 366, and ed. 2, vol. ii. (1887), pp. 227-234. 



§ ' Geoguostische Beschreibung des Konigreiehs Bayern, Abth. iii. Geogu. 

 Beschr. des Fichtelgebirges,' pp. 213-218. Gotha, 1879. 



